The present invention is directed toward a fiberizing spinner for converting molten material into a plurality of fibers and more particularly toward an ultrahigh velocity water-cooled copper spinner.
The function of a fiberizing spinner is to impart kinetic energy to a stream of molten material so that high velocity air, steam or other vapor, impinging on the rapidly moving stream of molten material, can force that stream into a multiplicity of small diameter fibers of considerable length. Such spinners are used, for example, in the manufacture of mineral or "rock" wool fibers for thermal insulation.
For the long fiber, low density mineral wool in general use in the U.S. for attic insulation, single wheel spinners have proved quite effective. For higher density fiber for cavity wall retrofit or industrial pipe covering, ceiling tiles, etc., it is more customary worldwide to use the so-called "four-wheel" spinner.
This four-wheeled spinner consists of 4 parallel, powered spindles each terminating in a water-cooled wheel of 10 to 14 inches in diameter with a 4 to 5 inch rim width, all 4 wheels being mounted in the same plane so that molten material dropping onto the top wheel is given velocity and slung onto the second wheel, and so on. Past the lowest, or third and fourth wheels, high velocity air or steam pushes the now highly energetic molten material laterally with such force that it separates into fibers.
These 4 spinner wheels have in the past been constructed of steel, conventionally water-cooled. As a result, the wheels are worn out by approximately one week's work and have to be refaced at considerable expense.
As explained more fully in Applicant's U.S. Pat. No. 4,032,705 (the entire subject matter thereof being included herein by reference), Applicant has discovered that the rapid, consistent removal of large quantities of energy (in the range of 1 BTU per square inch per second) through a water-cooled metal barrier, without damage to that barrier, requires that the metal have excellent thermal conductivity and a reasonably high melting point, and be force-cooled at a constant temperature by the creation and efficient removal of steam at its back face.
Converting 1 pound of water into steam requires 967 BTU's of heat at 212.degree. F. (or 536 calories per gram at 100.degree. C.). If water can be made to present itself consistently to the area to be cooled and there to turn into steam, and then to leave the area immediately to make room for more water to arrive, a highly efficient and predictable cooling system results. The area to be cooled must, of course, be kept free of accretion to obviate the film effects which are adverse to efficient thermal transfer.
Experimentation has shown that the best way to remove the steam film as rapidly as it forms is by applying ultrahigh velocity cooling water to the back surface of the metal barrier. A cooling water velocity of at least 10 feet per second has proved to be required, and this velocity must be at the surface of the metal, not merely at the center of a substantial cooling passage of which the metal barrier is one of the walls. The preferred water cooling velocity is at least 20 feet per second. It should be readily apparent that such velocities require high flow rates through small passages, thereby generating pressure drops of the order of 20 to 60 psi, depending on the surfaces, shapes and length of the area to be cooled.
To enhance the effectiveness of this cooling, a readily workable metal of reasonable cost and melting point and high thermal conductivity is required. From a table of the physical properties of the elements, a selection of an easily workable, relatively inexpensive material with a melting point about 1,000.degree. C. and good thermal transfer capability results in the following list:
______________________________________ MELTING POINT CONDUCTIVITY ELEMENT (.degree.C.) (calgmcm/sqcm/sec/.degree.C.) ______________________________________ Chromium (Cr) 1875 0.16 Copper (Cu) 1083 0.943 Iron (Fe) 1537 0.18 Molybdenum 2610 0.34 (Mo) Nickel (Ni) 1453 0.22 Silver (Ag) 960 1.00 (for comparison) ______________________________________
Chromium, molybdenum and nickel are not really easily workable and they are relatively expensive. Furthermore, these materials have thermal conductivities which are from 3 to 5 times poorer than that of copper.
Because of the relatively low melting point of copper and the corresponding higher melting point of iron, the automatic and quite incorrect choice in the past for a water-cooled spinner has been steel. This has been true even though it has a thermal transfer ability less than 1/5 that of copper. Furthermore, for a number of reasons, the water-cooled steel has a tendency to form films thereon of a highly insulating nature.
Compounding this technical felony is the fact that, to Applicant's knowledge, no attempt has been made to ensure the efficient removal of heat energy from the back face of a spinner by the encouragement of steam formation, against a clean surface, made effective by the immediate removal of that steam by new cooling water moving at "ultrahigh velocity." It should be pointed out that the use of stainless steel only makes matters worse since stainless steel grades have thermal transfer abilities 16 to 24 times poorer than copper.